EP0636528B1 - Electric power steering apparatus - Google Patents

Description

BACKGROUND OF THE INVENTION Field of the Invention
• The present invention relates to an electric power steering apparatus which employs an electric motor to provide power to assist the steering force for a vehicle.
Description of Related Art
• The electric power steering apparatus serves to detect the steering torque and to assist the steering force by the rotation of an electric motor to be driven according to the detected torque.
• FIG.1 is a block diagram showing an outline construction of the electric power steering apparatus. The output of a torque sensor 2 provided on a steering shaft (not shown) is inputted through an interface 3 into a CPU 1, wherein first the A/D conversion (11) is carried out, then the phase compensation (12) performed. The output of a vehicle speed sensor 7 is also inputted into CPU 1. In a memory of the CPU 1, there is stored a table (13) of the relations between the detected torques and vehicle speeds and the target current values of the drive current for an electric motor 5, and the target current corresponding to the inputted torque and vehicle speed is read out. Also, the output of A/D conversion is differentiated (18), and this result is added to the read-out target current (14). The addition of the differentiated value is for the purpose of the inertial compensation of the motor 5.
• In the CPU 1, the deviation is calculated (15) between the added result and the feedback drive current of the motor 5 which is detected by a drive current detector 6 and the A/D conversion (19) is carried out in the CPU 1. The deviation is dealt in the PI calculation (16), thereby the control variable is calculated, and the PWM wave signal corresponding to the variable and the signal indicating the rotary direction of the motor 5 are made out (17). Those signals are given to a driving circuit 4 which is constructed as an H-type bridge of four switch transistors together with the motor 5.
• FIG.2 is a flow chart showing the operating steps of the conventional PI calculation (16) in the CPU 1. The difference ΔI_n is obtained between the target current I_M as the added result 14 and the detected driving current I_s of the motor 5 (S1). Next, the integral element I_n is calculated (S2) as follows; $${\text{I}}_{\text{n}}{\text{= ΔI}}_{\text{n}}{\text{x K}}_{\text{I}}{\text{(K}}_{\text{I}}\text{: constant)}$$

  

  Next, the difference ΔP_n is calculated between ΔI_n and the former difference ΔI_n-1 (S3) as follows; $${\text{ΔP}}_{\text{n}}{\text{= ΔI}}_{\text{n}}{\text{- ΔI}}_{\text{n-1}}$$

  

  then, the proportional element P_n is calculated as follows (S4); $${\text{P}}_{\text{n}}{\text{= ΔP}}_{\text{n}}{\text{x K}}_{\text{P}}{\text{(K}}_{\text{P}}\text{: constant)}$$

  

  And, the target voltage value V_M to be given to the motor 5 is calculated (S5) as follows; $${\text{V}}_{\text{M}}{\text{= {(V}}_{\text{O}}{\text{+ I}}_{\text{n}}{\text{+ P}}_{\text{n}}{\text{) + V}}_{\text{O}}\text{}/2}$$

  

  then, the data are renewed (S6). After this, the duty of the PWM wave required to realize the target voltage is calculated.
• By the way, the motor 5 is an inductive load type, so that the current does not lead to flow when the pulse of the PWM wave is small on duty. For reasons such as the capacity of the CPU 1, the PWM wave is employed in a frequency of 20 kHz or so. In this frequency, the current can flow in the motor more than 50% of the duty. Then, the constants K_I, K_P are set up so that the stable control may be performed more than 50% of the duty. As a result, there was a problem that the response was too late until the duty reached over 50% at the rise of the motor current.
• This is because I_M and I_S are all small, ΔI_n is small, and it is required that the operating steps of S1 to S6 are repeatedly carried out a considerable times until V_M or the duty becomes a certain large value. Owing to this poor responsibility, there was a problem that when the steering was repeated right and left, the assistance force was behind the steering force, and the operator felt vibration.
• EP-A-0,522,466 discloses a controlling apparatus for a power steering system which has a differentiation unit in which a signal selecting element judges whether a first torque detecting signal T₁ having a high gain is in a saturated range or not, outputs as a differential current I_d a first differential current I_d1 obtained by differentiating the first torque detecting signal T₁ in case where the first torque detecting signal T₁ is not in a saturation range, and outputs as the differential current I_d a second differential current I_d2 obtained by differentiating a second torque detecting signal T₂ having a low gain in the case where the first torque detecting signal T₁ is in the saturation range for adjusting a control value of a driving current of a steering assisting motor.
SUMMARY OF THE INVENTION
• An object of the present invention is to provide an electric power steering apparatus wherein the responsibility is improved by forcibly increasing the former target voltage, for example, to the value equivalent to 50% of the duty under the predetermined conditions, and the vibration of the steering wheel is prevented.
• According to the present invention, this object is achieved by the electric power steering apparatus as defined in independent claim 1. Embodiments of the invention are disclosed in the dependent claims.
• In the electric power steering apparatus of the invention, an electric motor for assisting the steering torque is adapted to be driven by PWM wave, and the duty of the PWM wave is adapted to be under the feedback control in time series by detecting the driving current of the motor. The apparatus is provided with a first means for judging whether predetermined conditions are fulfilled or not, and a second means for setting the former target voltage of the motor for the feedback control to the voltage value decided in reference to the minimum duty of the PWM wave for driving the motor when the above-described conditions are fulfilled by the first means. The conditions are defined that the direction of the target current in this time is switched from that in the former time or that the detected value of the driving current of the motor is zero.
• Even when the current is rising, the former target voltage, and therefore this time target voltage is immediately set to the value equivalent to the high duty, so that the motor can be driven in short order, the responsibility is improved, and accordingly the vibration does not occur even when the steering is repeated right and left.
• The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG.1 is a block diagram of an electric power steering apparatus;
• FIG.2 is a flow chart of the conventional PI calculation;
• FIG.3 is a flow chart of the PI calculation of the invention;
• FIG.4 is a flow chart of the PI calculation of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
• Hereinafter, the present invention will be described according to the drawings showing its embodiments.
• The hardware construction of the electric power steering apparatus according to the present invention is as shown in FIG.1. And, the difference from the prior art is in the operating steps relating to the PI calculation 16 in the CPU 1. FIG.3 and 4 are flow charts showing those operating steps.
• It is judged whether the target value in this time I_M = 0 or not (S11) with regard to the driving current of the electric motor to be obtained according to the added result 14. In the case of I_M = 0 , the operation is transferred to the step S1 for seeking the conventional control target ΔI_n.
• In the case of I_M ≠ 0, it is judged whether the direction of the former target current I_O and that of this time target current I_M are same or not (S12). When the directions are same, it is judged whether the detected value I_S = 0 or not (S16) with regard to the driving current of the electric motor 5. When I_S ≠ 0, the operation is transferred to the step S1. That is, when this time target current I_M = 0 , or when the directions of the steering are same (the direction of the target current is not changed) and moreover I_S is not 0 (already the current has flown), the decision of the target current V_M may be performed in the conventional manner (S1 to S5).
• On the contrary, when the direction of I_O and that of I_M are different (when the direction of the detected torque is changed), or when I_S = 0 (when the current is about to flow), the operation is transferred to the next step S13, where the former target voltage V_O is set in a value equivalent to 50% duty of the PWM wave (S13). This value may be properly determined according to the requirement for the highness of the responsibility. Accordingly, it is set that ΔI_n-1 = 0 (S14) and I_S = 0 (S15). After that, the operation is transferred to the step S1. That is, in such a case, there becomes the state of the minimum duty of the PWM wave where the motor 5 may substantially be driven. And moreover, such a situation is forcibly made that the motor 5 would not be driven in the former state.
• The target voltage obtained V_M in the step S5 is employed in the PWM wave calculation 17. After this step S5, it is judged whether the direction of V_M and that of I_M are same or not (S51). In the case of the same direction, the operation is transferred to the step S6. In the case of the different directions, V_M = 0 is set (S52). This serves to drive the motor 5 only in the direction of the target current I_M, and not to drive the motor 5 when I_M and V_M are different in the directions. Then, for the next calculation, the data are renewed such that V_M to V_O and Δ I_n to ΔI_n-1 (S6).
• In the present invention as described above, since the duty of the PWM wave is forcibly increased to the value relating to the minimum value for driving the motor when the direction of the steering is switched, or when the motor is about to be driven, the motor can be rapidly driven in the direction required for the assistance, and accordingly the generation of any vibration may be prevented even when the steering to right and left is repeated.
• As this invention may be embodied in several forms without departing from the terms of the claims. Therefore, the invention is defined by the appended claims rather than by the description preceding them.

Claims (3)

1. An electric power steering apparatus, in which an electric motor (5) for assisting the steering torque is adapted to be driven by PWM wave, and the duty of the PWM wave is adapted to be under the feedback control in time series by detecting the driving current of the electic motor (5), said apparatus comprising first means (16, S12, S16) for judging whether predetermined conditions are fulfilled or not, characterized in that it further comprises second means (16, S13) for setting the former target voltage of the electric motor (5) for the feedback control to the voltage value decided in reference to the minimum duty of the PWM wave for driving the electric motor (5) when the predetermined conditions are fulfilled by said first means (16, S12, S16), wherein said first means (16, S12) judges whether the direction of the target current in this time for the feedback control is switched or not from that of the former time.
2. An electric power steering apparatus as set forth in Claim 1, wherein said first means (16, S12, S16), when judging that the direction of the target current in this time for the feedback control is not switched from that of the former time, judges whether the detected value of the driving current in the electric motor (5) is zero or not.
3. An electric power steering apparatus as set forth in Claim 1, wherein the minimum duty of the PWM wave for driving the electric motor is about 50%.

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